Nobody tell Trump and his crusade against environmental preservation, but the concept of different blends of gasoline based on the season was introduced by the Environmental Protection Agency about 36 years ago. The gasoline you put in your gas tank is a liquid, but gas has a low volatility point, or the temperature at which a liquid evaporates or vaporizes. According to Car and Driver, the EPA began restricting the volatility of retail gasoline in the summertime before the 1990 Clean Air Act in 1989 to minimize the amount of residual gas vapor that escapes from cars fuel systems in higher temperatures.
As C/D explains, when gasoline vaporizes, it contributes to the brownish smog that you frequently see surrounding major metropolitan areas like the Los Angeles region. Dense gas vapors are heavier than air, so they fall to the ground and contribute to ground-level ozone concentrations, which are a major contributor to smog levels. Other factors that contribute to ozone include tailpipe emissions, factories, and smokestacks. Ozone forms when nitrogen oxides (NOx) or other volatile organic compounds (VOCs) react with sunlight, making summer a particularly dangerous time for air pollution.
Smog can be dangerous
According to the American Lung Association, ozone reacts with lung tissue and aggressively attacks it. When humans breathe smog, immediate breathing problems may occur including shortness of breath, wheezing, coughing, and asthma attacks, and long-term exposure increases the risk of respiratory illnesses, metabolic disorders, nervous system issues, and reproductive issues.
The difference between summer and winter gasoline blends lies in its Reid vapor pressure, or RVP. Federal law prohibits gasoline with an RVP greater than 9 psi to be sold at retail stations from June 1 through September 15. Some states and major metropolitan regions have different standards due to factors like population density and weather. California has a 7-psi limit for most fuels and a 6.9 psi-limit for nonoxygenated fuels, so low-RVP gasoline must be sold as early as April 1 and as late as October 31. The sunny desert metropolis of Phoenix, AZ also extends its summer-grade fuel requirements into the fall.
What changes between grades?
Fuel prices in the summertime are often higher than in winter, and that’s due in part to the different components of the blends. One difference in the refining process that produces the summer blend of gasoline with its low RVP is a reduction in the amount of butane added to the mixture. Butane is a cheaper chemical that when added to gasoline helps lower cost, but it has a very high RVP of 52 psi, which raises the RVP of the gasoline it’s added to, thus making it unfit for summertime use.
Instead of adding cheap butane to summer blends of gasoline, refiners use more expensive additives which can lengthen the refining process and reduce gasoline yields including alkylates and reformates to keep the RVP levels lower and more suitable for warmer temperatures. Some states, counties, and regions have different “Boutique Fuel Programs” to meet their specific regional needs, as explained by this chart provided by the Environmental Protection Agency.
Ultimately, the different summer and winter blends of gasoline are in place to protect American citizens’ right to clean air regardless of their region’s population density and weather conditions. As a native Angeleno, I’m grateful for California’s more stringent standards because our smog levels and air quality are bad enough as it is. My mom who’s also a native Angeleno remembers having “smog days” when they were a kid where the smog was so bad that schools didn’t let kids on the playground at recess, so I’m glad we don’t face those smog levels anymore.